I think the one big thing for me would be Evangelion, movie and series. Both out of nostalgia, and because between the human drama, giant robot fights, and visual WTFery there are a lot of videos you can make with it. I don't have four others that I couldn't do without.

I rarely use a series more than once, but without Naruto, I might not have become a AMV editor in the first place (although I don't plan on using that source ever again)

The Noein AMV that I made in 2009 got me my current position of MomoCon's AMV Director, and was the very last vid that I made in WMM. Also, the Clannad AMV that I made in that same year won me my very first AMV award.

And Zatch Bell is the only series that I've made 3 full length AMVs with.

I...don't...know...There are some sources I do enjoy editing with (Mushi-shi, 5 Centimeters Per Second) but there are quite a few other sources I'd love to edit with and just never had a concept for. Honestly, I don't think I have any source that I can't live without.